Many swimming pools have a pump that recirculates the water through pleated paper filter cartridges. These filters commonly consist of a parallel array of cylindrical elements with two open ends. The filter elements are periodically removed from the filter housing, manually cleaned (e.g., by immersion in a chlorine-containing solution, spraying with a garden hose, or brushing), and visually inspected to see if they are worn to the point of requiring replacement.
Some swimming pools have sand filters that are cleaned in situ by backwashing. In situ cleaning is commonly practiced in non-swimming pool filtration applications in which the filtered medium is something other than water (in which case ex situ open washing of the filter with clean filtered medium could be expensive and, in many cases, would spill contaminants), or in which the filter system is very large. Numerous inventors have provided apparatus and methods in this area. Notable among these teachings are:
U.S. Pat. No. 3,674,151, wherein Freeland discloses a filter screen backflushed by a set of cleaning nozzles. A corresponding set of conduits on the upstream side of the filter rotate with the nozzles and carry away whatever dirt is washed off the screen. PA1 U.S. Pat. No. 4,271,018, wherein Drori teaches a filter that is rotated so that the entire surface can be backwashed with a fixed nozzle. PA1 U.S. Pat. No. 4,923,600, wherein Krofta teaches backwashing a filter by rinsing its downstream side with an array of nozzles that move relative to the filter medium. Krofta also uses a rotating brush to clean the upstream side of the filter. PA1 U.S. Pat. No. 4,941,971, wherein Albright teaches apparatus for in situ cleaning of a swimming pool filter rotatably mounted in a container. In one embodiment, Albright provides a moving nozzle directed at the upstream side of the filter. This nozzle spins the filter fast enough that centrifugal force helps to flush contaminants off its outer surface. PA1 U.S. Pat. No. 5,108,592, wherein Wilkins et al., describe a set of backflushing nozzles inside a cylindrical filter screen and directed at the upstream side of the screen. Wilkins' screen is journaled onto a shaft and is rotated by the stream of cleaning fluid. PA1 U.S. Pat. No. 5,228,993, wherein Drori discloses several approaches to cleaning a composite filter comprising a stack of annular disk filters. Drori's apparatus includes an array of backwash nozzles located along the axis of the generally cylindrical composite filter. These nozzles rotate about the vertical axis, and are translated axially by an external handcrank or motor.